Fix creation of GML rings with unresolvable member references

Here's an example for such a beast:

```xml
<gml:Ring>
  <gml:curveMember xlink:type="simple">
    <gml:Curve xsi:type="aixm:CurveType" srsName="urn:ogc:def:crs:EPSG::4326" gml:id="uuid.d90d567d-d04d-45ac-bd17-6e033d126031">
      <gml:segments>
        <gml:GeodesicString interpolation="geodesic">
          <gml:posList>47.83222222222223 7.556666666666667 ...</gml:posList>
        </gml:GeodesicString>
      </gml:segments>
    </gml:Curve>
  </gml:curveMember>
  <gml:curveMember xlink:type="simple" xlink:href="urn:uuid:afac8ce6-c743-47d3-9584-8ff9a05df125"/>
  <gml:curveMember xlink:type="simple">
    <gml:Curve xsi:type="aixm:CurveType" srsName="urn:ogc:def:crs:EPSG::4326" gml:id="uuid.ffbf75d9-019e-430d-abd6-8ad6f2c86d23">
      <gml:segments>
        <gml:GeodesicString interpolation="geodesic">
          <gml:posList>47.58996944444445 7.589102777777778 ...</gml:posList>
        </gml:GeodesicString>
      </gml:segments>
    </gml:Curve>
  </gml:curveMember>
  <gml:curveMember xlink:type="simple" xlink:href="urn:uuid:e920501f-78c3-41f5-9b38-14713199e858"/>
</gml:Ring>
```

Before this patch, the reading of the above GML snippet would result in an exception in case the reference could not be resolved. After the patch, this is possible. The exception will only occur if there's access to  the of the members.